Rack Heating Issues

[meowmeowkazoo]

So I have a rack with four levels using 41 quart sterilite bins. It is made of 1" thick melamine. I have a herpstat connected to one of the middle levels to try and maintain a hot spot of 90 degrees. However, when I do this, the top two levels reach only 86 degrees, while the bottom two maintain the 90 degrees. It's driving me insane, how do I fix this?

[kitedemon]

What are your ambient room temps? Racks often are greatly effected by air temps.

Did you check all your connections?

Does the heat source run in one strip? If it does run in a single strip, does the top not have heat source along the bottom and side or just the bottom? That often can account for a bit of a difference. My rack I find the top is usually the warmest and bottom coolest but I have mine well insulated.
The next question is did you swap the thermometers? It is not a huge difference you may find that the two thermometers are not reading the same. Some have quite a lot of allowable error (accurites are +/-4ºF) this may account for the difference too.

In my set up mostly due to the cold room the rack is in I needed to use a probe on the top set and a probe on the bottom separately. My solution to my problem, not necessarily yours.

[meowmeowkazoo]

I usually keep the ambient temps around 80 as much as possible. The rack is near a window. The connections are all working. There are four separate pieces of heat tape that are parallel wired. I don't have two thermometers, just one.

I guess I could use the herpstat for two connections and then use a different thermostat for the other two levels. I can't afford another herpstat right now, but that might work.

[kitedemon]

Do you have an IR gun?

If you do just check the temp of the tape on each level. It might lend some insight. At close to 80º I can't see that much loss it doesn't make sense.